METHOD FOR EXTENDING THE LIFE OF A BATTERY

Abstract

One aspect of the invention relates to a method (100) for extending the service life of a battery comprising a plurality of cells, said method (100) comprising the steps, executed by a computer, of: determining (102) a force applied to a wall of at least one cell of the battery, determining (103) a maximum permissible state of charge of the at least one cell as a function of the determined applied force. Figure 3

Description

Title of the invention: METHOD FOR EXTENDING THE LIFE OF A BATTERY

[0001] The invention relates to the management of the charge of a battery, in particular the management of the charge of a battery for an electric or hybrid motor vehicle.

[0002] Such batteries generally comprise a plurality of electrical accumulators, also called cells. Each cell includes an electrochemical system capable of being recharged up to a maximum open-circuit voltage.

[0003] Batteries are generally controlled by an electronic battery management system, more commonly known as a BMS (Battery Management System), which manages, for example, the battery charging phases to bring the battery to the desired voltage at the end of charging without causing excessive heating and preventing any one cell from reaching a significantly higher or lower charge level than the other cells in the battery. The BMS can be configured to calculate a dimensionless variable, such as a State of Charge (SOC), which quantifies the battery's charge level using a value between zero and 1.

[0004] The BMS can also be configured to estimate, during vehicle operation, a State of Health (SOH) value, which is a coefficient used to quantify the level of energy available in the battery once it is fully charged, taking into account the degradation of battery performance over its life cycle. The SOH value can be calculated using various methods and allows for estimating the energy available in the battery at the end of the charge and the mileage the driver can therefore expect to travel.

[0005] A method for managing a battery of accumulators is known from document FR-B1-3009093, enabling the precise determination of the aging state of a battery equipping an electric or hybrid vehicle. This method also optimizes the management of the battery charging phases based on the calculated aging state. More specifically, to improve battery life, the BMS imposes, at the beginning of the battery's life cycle, a maximum voltage at the end of charging that is lower than the maximum acceptable voltage at the end of charging. The BMS then increases this maximum voltage during the battery's life cycle. This management of the charging phases aims to reduce the aging rate as a function of battery usage.

[0006] However, as soon as the battery's state of aging reaches a level of Once the aging threshold is reached, the BMS system prohibits any battery recharging for safety reasons.

[0007] The object of the invention is to overcome the disadvantages of the prior art by proposing a method to increase the usage time of a battery beyond the threshold aging level.

[0008] In this context, the invention thus relates, in its broadest sense, to a method of extending the service life of a battery comprising a plurality of cells.

[0009] The method comprises the steps, executed by a computer, of: • determine the force applied to the wall of at least one battery cell, • determine a maximum permissible load state of at least one cell as a function of the determined applied force.

[0010] It should be noted that the more advanced the aging state of a cell, the greater the force applied to its wall. When the applied force is too great, the cell is no longer usable. Similarly, the state of charge contributes to increasing this force. Thanks to the invention, the maximum permissible state of charge is defined as a function of a specific applied force on the cell wall. Thus, it is possible, for example, to reduce the overall force, formed by the specific applied force plus the additional force generated by the maximum permissible state of charge, by reducing the maximum permissible state of charge. The invention therefore makes it possible to use batteries even at higher ages.

[0011] In addition to the characteristics mentioned in the preceding paragraph, the process according to the invention may have one or more additional characteristics from among the following, considered individually or according to all technically possible combinations.

[0012] According to a non-limiting aspect of the invention, the method includes a step of determining an aging state of the cell, the applied force being determined as a function of the determined aging state of the cell.

[0013] According to a non-limiting aspect of the invention, the maximum permissible load state is determined so as to apply on the cell wall an additional force due to a cell expansion less than a predetermined threshold force - the applied force determined.

[0014] According to a non-limiting aspect of the invention, the predetermined threshold force is between 20kN and 30kN.

[0015] According to a non-limiting aspect of the invention, the predetermined threshold force corresponds to a cell aging state between 60% and 80% of a new cell state.

[0016] According to a non-limiting aspect of the invention, the determined aging state is a function • of a ratio between a maximum amount of electricity that can be stored in the cell at a given moment and a maximum amount of electricity that can be stored in the cell when it is new; • the number of cell recharges performed; • an integration of the number of hours of cell use multiplied by a coefficient dependent on a measured cell temperature; or • of an accumulation of energy charged or discharged from the cell.

[0017] Another aspect of the invention relates to a computer arranged to communicate with a battery comprising a plurality of cells, the computer being further arranged to implement the steps of the process according to any one of the aforementioned aspects of the invention.

[0018] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures.

[0019] [Fig-1] schematically illustrates a lithium-ion type battery module according to the state of the art.

[0020] [Fig.2] shows, schematically, in particular a control unit of a lithium-ion type battery according to a non-limiting aspect of the invention.

[0021] [Fig.3] illustrates a step diagram of a non-limiting implementation method of the method according to the invention.

[0022] [Fig.4] shows a graph illustrating the forces applied to a cell of a lithium-ion type battery.

[0023] Figure 1 schematically illustrates a module 1 comprising a battery cell 2, for example of the lithium-ion type. For the sake of simplicity, Figure 1 illustrates a single cell 2, but it is understood that a module 1 may comprise several battery cells 2.

[0024] This cell 2 has a wall 3 containing a negative electrode 4 and a positive electrode 5 separated from each other by means of separators 6. This cell 2 also contains an electrolyte 7.

[0025] The elements contained in the wall 3, forming an envelope, must be kept in contact within a range of positive force.

[0026] To combat the internal pressure which tends to push back the wall 3 of the cell 2, a frame 8, typically made of aluminium, encloses the cells 2, only one is visible in the figure.

[0027] During the use of cell 2, a secondary reaction occurs. This secondary reaction generates a passivation layer 9 on the surface of the negative electrode 4. This passivation layer 9 is more commonly known as the interphase between the electrolyte and the surface or SEI (for Solid-electrolyte interphase in English).

[0028] This passivation layer 9 increases the volume of the negative electrode 4. This increase in volume can reach 4% of the initial volume of the cell 2.

[0029] During this secondary reaction, gases are also produced which contribute to an increase in the internal pressure of cell 2, which, for safety reasons, must remain perfectly sealed. This pressure can reach 6 bar.

[0030] The more advanced the aging state of cell 2, the greater the force exerted on the wall 3 of cell 2. The aging state of a cell is therefore very strongly linked to the force applied to the wall 3 of cell 2 generated by its expansion.

[0031] Fig. 2 illustrates a calculator 10, for example formed by a battery control unit 11 comprising a plurality of cells 2. This battery control unit 10 is better known as the BMS (for "Battery Management System").

[0032] In this non-limiting embodiment, the cells 2 are contained within a frame 8 and together form a module. The battery 11 may comprise several modules.

[0033] The battery control unit 10 communicates with a control unit 12 of an electric motor 13 and an electric charger 14.

[0034] Thus, the battery control unit 10 is arranged to allow or not the charging of the cells 2 of the battery 11.

[0035] The battery control unit 10 is arranged to implement the steps of a method for extending the usage time of a battery according to the invention.

[0036] Figure 3 shows a step diagram of an implementation method of process 100 according to the invention.

[0037] The steps of the process 100 are executed by a computer such as, for example, the battery control unit 10 shown in [Fig.2].

[0038] The implementation of the different steps of process 100 is also illustrated in support of [Fig.4].

[0039] In particular, [Fig.4] illustrates a force applied in kN on the wall 3 of a cell 2 as a function of an aging state SOHc in percent.

[0040] A first curve Cl illustrates a force applied to the wall 3 of a cell 2 as a function of an aging state SOHc of the cell 2 + a charge state of 0%. The charge state is a state of charge of type SOC (for State Of Charge).

[0041] A second curve C2 illustrates a force applied to the wall 3 of cell 2 as a function of an aging state SOHc of cell 2 + of a maximum permissible charge state SOC of 100%.

[0042] A third curve C3 illustrates a force applied to the wall 3 of cell 2 as a function of an aging state SOHc of cell 2 + a variation of a state of SOC load between 0% and 100%.

[0043] A fourth curve C4 illustrates a curve representing a variation of a maximum SOC state of charge in percent.

[0044] The method 100 includes a step, performed by the battery control unit 10, of determining 101 an aging state of at least one cell 2.

[0045] In a non-limiting example, the state of aging can be represented by a SOHc (State of Health capacity). Such a state of aging reflects the number of ampere-hours that cell 2 can store at a given time.

[0046] The determined state of aging of cell 2 can, for example, be a function of: • a ratio between a maximum quantity of electricity that can be stored in cell 2 at a given time and a maximum quantity of electricity that can be stored in cell 2 in its new state, • of the number of cell 2 recharges performed, • an integration of the number of hours of cell 2 usage multiplied by a coefficient depending on a cell temperature measured by a temperature sensor, or • of an accumulation of charged or discharged energy of cell 2.

[0047] The method 100 includes a step, executed by the battery control unit 10, of determining 102 a force applied to the wall 3 of the cell 2.

[0048] In a non-limiting implementation, the applied force is determined according to the aging state of cell 2 determined during the previous step 101.

[0049] Indeed, during the use of cell 2, a secondary reaction occurs generating a passivation layer 9 on the surface of the negative electrode 4 of cell 2. This passivation layer 9 increases the volume of the negative electrode 4 and generates an increase in the force applied to the wall 3 of cell 2. This increase in volume, and therefore the increase in the force applied to the wall 3 of cell 2, is irreversible and is linked to its SOHc aging state.

[0050] The method 100 includes a step, executed by the battery control unit 10, of determining 103 a maximum permissible state of charge of the cell 2 as a function of the determined applied force.

[0051] The maximum permissible load state is determined so as to apply on the wall 3 of cell 2 an additional force due to a dilation of cell 2 less than or equal to a predetermined threshold force - the applied force determined.

[0052] When a force exceeding the threshold force is applied to the wall 3 of cell 2, it is forbidden to use the battery 11 containing it. Indeed, beyond this threshold force, cell 2 risks rupturing or catching fire.

[0053] In the non-limiting example illustrated in [Fig.4], the threshold force is 25kN.

[0054] Thus, in this embodiment example, from a determined applied force At 20 kN, corresponding to a 20% SOHc aging state of cell 2, the maximum permissible charge state (C4 curve) is reduced so that the applied force determined due to the aging state of cell 1 (C1 curve) + the additional force due to expansion applied to the wall 3 of cell 2 due to the maximum permissible charge state do not together exceed the threshold force of 25 kN.

[0055] The battery control unit 10 thus limits the battery charging current from the electric motor 13 or the charger 14 by transmitting a current limiting information to the controller 12.

[0056] This non-limiting implementation allows the use of cell 2, and therefore battery 11, to be extended beyond 80% of its aging state by limiting the maximum permissible state of charge (SOC). Indeed, this reduction, illustrated by the fourth curve C4, of the maximum permissible state of charge (SOC) from 20 kN, makes it possible, even though the aging state SOHc is at an advanced stage, to limit the overall force (curve C2), formed by the force due to the aging state plus the force due to the maximum permissible state of charge (SOC), applied to the wall 3 of cell 2 below the critical threshold of 25 kN in the example.

[0057] Without this reduction, it can be seen on the dotted portion of the second curve C2 that the overall force applied to the wall 3 of cell 2, a function of the force due to the aging state SOHc of cell 2 and that due to the maximum permissible state of charge SOC, would exceed the threshold force of 25 kN. Such a situation would be unacceptable. Thus, the battery 11 would no longer be used once 20% of the aging state SOHc had been reached.

[0058] It should be noted that a person skilled in the art is able to make different variations to the aforementioned aspects of the invention, for example by modifying the value of the threshold force.

Claims

Demands

1. A method (100) for extending the service life of a battery (11) comprising a plurality of cells (2), said method (100) comprising the steps, executed by a computer (10), of: - determining (102) a force applied to a wall (3) of at least one cell (2) of said battery (11), - determining (103) a maximum allowable state of charge (SOC) of said at least one cell (2) as a function of said determined applied force, characterized in that it comprises a step of determining (101) a state of aging (SOHc) of the cell (2), said determined applied force being determined as a function of said determined state of aging (SOHc) of said cell (2).

2. Method (100) according to claim 1, characterized in that the maximum allowable state of charge (SOC) is determined so as to apply on the wall (3) of the cell (2) an additional force due to a dilation of said cell (2) less than a predetermined threshold force - the determined applied force.

3. Method (100) according to the preceding claim characterized in that the predetermined threshold force is between 20kN and 30kN.

4. Method (100) according to claim 2 or 3 characterized in that the predetermined threshold force corresponds to an aging state (SOHc) of the cell (2) between 60% and 80% of a new state of the cell (2).

5. Method (100) according to any one of claims 1 to 4, characterized in that the determined state of aging is a function of a ratio between a maximum quantity of electricity that can be stored in the cell (2) at a determined instant and a maximum quantity of electricity that can be stored in the cell (2) in a new state.

6. Method (100) according to any one of claims 1 to 4, characterized in that the determined state of aging is a function of a number of cell (2) recharges carried out.

7. A method (100) according to any one of claims 1 to 4, characterized in that the determined aging state is a function of integrating a number of hours of cell (2) use multiplied by a co- efficient depending on a measured cell temperature.

8. Method (100) according to any one of claims 1 to 4, characterized in that the determined aging state is a function of an accumulation of energy charged or discharged from the cell (2).

9. Computer (10) arranged to communicate with a battery (11) comprising a plurality of cells (2), said computer (10) being characterized in that it is further arranged to carry out the steps of the method (100) according to any one of the preceding claims.

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